1. Field of the Invention
This invention relates to infrared radiators capable of emitting heat rays in the range of infrared ray wavelengths by application of heat.
2. Description of the Prior Art
Infrared rays are more readily absorbed by materials to be heated as compared with visible light rays having wavelengths of 0.3 to 0.8 .mu.m and activate the molecular movement of the materials with the attendant great effect of heat generation. Accordingly, the infrared rays have widely been used in the fields of heating and drying.
It is well known that the transmission of heat energy can be classified into three categories of conduction, convection and radiation.
Particular reference is now made to cooking devices. Cooking of food has conventionally been conducted by various manners including, for example, methods chiefly using direct thermal conduction in which food is roasted or broiled by direct flame such as from gases, petroleum or solid charcoal or done on heating plates such as a hot plate, and methods in which air such as in ovens is heated and the heat energy from the heated air is transmitted to cooking food, i.e. the heating mainly depends on convection.
Components constituting foods are comprised of water, proteins, starch, fats and the like, and these materials show absorption characteristics as shown in FIG. 1, i.e. they have great absorption factors or absorptivities in the range of infrared wavelengths, particularly in the range of far infrared wavelengths above 3 .mu.m and have such properties as to absorb the infrared energies corresponding to the absorption factors of the individual constituents and convert them into heat. In order to more effectively heat foods, it is necessary to irradiate from outside a great deal of infrared rays having wavelengths corresponding to the absorptivities of the individual constituents of food.
By the irradiation of the infrared rays, the molecules of the constituents of a material to be heated are vibrated and self-heated, so that this radiation heating shows better heat and energy efficiencies than the conventional conduction and convection methods, with the attendant advantage that the energy can be saved.
In order to effectively heat cooking stuffs, the infrared heating is favorable as will be seen from the absorption characteristics of FIG. 1. To this end, there is needed a heating source for radiating the infrared rays of wavelengths corresponding to the wavelengths absorbed by the cooking stuff.
As regards heating, human body is constituted of water, proteins, fats and the like. Similarly to cooking stuffs, effective heating of human body is conveniently feasible by the infrared heating as is apparent from the absorption characteristics of human body shown in FIG. 2.
In general, the energy E radiated from body is represented according to the Stefan-Boltzman equation: EQU E=.epsilon..sigma.T.sup.4 . . . (1)
in which .epsilon. represents an emissivity, .sigma. represents a constant, and T represents an absolute temperature .degree.K.
That is, the energy is determined by the temperature of body and the emissivity or radiation rate of material and thus it is possible to make an infrared radiation source by providing a material having high emissivity in the region of infrared wavelengths and heating it at a suitably high temperature.
It is known that materials exhibiting great values of .epsilon. of the equation (1) include ceramic materials. In fact, ceramic materials have conventionally been used as the infrared radiation source. That is, ceramic materials have been employed as radiator by depositing on substrate or by making sintered masses of ceramics by the following methods.
(a) Method in which ceramics are sintered at high temperatures to give ceramic sintered masses.
(b) Method in which ceramic layer is formed by flame spray coating.
(c) Method in which organic or inorganic binders are combined with ceramic materials and the mixture is applied and sintered.
Infrared radiators which are obtained by the method (a) using ceramic sintered masses are commercially available, for example, as Dschwamk burner employed in gas fittings. This is a system which includes a hot plate made of sintered ceramic having a multitude of fine through-holes made vertically of the plate surface, by which on combustion of gas beneath the hot plate, the flame passes through the fine through-holes whereupon the hot plate is heated thereby generating a great deal of infrared rays. However, this system has a number of disadvantages that the sintered ceramic mass is poor in mechanical impact strength and resistance to cold-to-hot heat cycle and also in productivity and economy, that the sintered ceramic mass is thick and large in weight, so that the heat capacity becomes great, leading to the slow rise of temperature at the initial stage of heating, and that because of the adiabatic property of the sintered ceramic mass, the surface temperature becomes low with a small radiation energy E of the equation (1). In other words, the sintered ceramic mass has a drawback that the radiation energy is small for the heating energy.
The spray coating method (b) is a method in which a metal surface is roughened such as by blasting and then ceramic materials are spray coated by the plasma or flame spray coating technique to form a spray coated layer or radiator layer. One of features of the ceramic radiator layer obtained by the spray coating technique resides in that the layer thickness is sufficient to be in the range of several tens .mu. to several hundreds .mu. and thus the heat capacity becomes so small that the ceramic layer is readily turned higher in surface temperature than the sintered ceramic mass system, with the attendant advantage that the radiation energy becomes great according to the equation (1). In this connection, however, the spray coated layer is formed by applying ceramic particles of high temperature on a metal substrate, so that the layer is substantially porous. Because of this porosity, the substrate is susceptible to an influence of corrosive environment and practical application of this type of radiator over a long time will cause the spray coated layer to be separated with a loss of the infrared radiating effect.
The method (c) using heat-resistant paints is as follows: Heat-resistant paints and infrared radiating materials are mixed together to give paints, which are then applied on a metal substrate and baked to form a film containing the infrared radiating material. However, with the arrangement mentioned above, the effective infrared rays emitted from the infrared radiating material is intercepted by the film. The reason for this is as follows: The main component constituting the heat-resistant paint is usually made of silicone resin, which shows a great absorptivity in the infrared wavelength range of 7 to 10 .mu.m. Accordingly, infrared rays in a certain range of wavelengths emitted from the infrared radiating material are filtered and there cannot be obtained infrared rays in the range of wave lengths effective for cooking stuffs and human body, resulting in a loss of energy and giving an adverse influence on the cooking performance and heating effect.